The overall goal of these ongoing studies is to define the structure/function relationships of a family of paired immunoglobulin-like receptors of activating (PIR-A) and inhibitory (PIR-B) isoforms in mice. PIR-A of approximately 85 kD and PIR-B of approximately 125 kD are cell surface glycoproteins having similar extracellular regions but with distinctive transmembrane and cytoplasmic regions. PIR-B is encoded by a single gene and contains three tyrosine-based inhibitory motifs in its cytoplasmic tail. In contrast, PIR-A receptors are encoded by multiple Pira genes and associate non-covalently with an adaptor protein, the Fc receptor common gamma chain (FcRgammac), to form a cell activation complex. In addition, the recently identified, heavily glycosylated PIR-A4 receptor of approximately 110 kD is expressed on the surface of FcRgammac-deficient cells. PIR-A and PIR-B are expressed by many hematopoietic cell types, including B cells, monocyte/macrophages, dendritic ceils, granulocytes, mast cells, and megakaryocyte/platelets, but not by T and NK cells. The cell surface levels of PIR increase as a function of cellular differentiation and activation. Based on our preliminary data, we propose the overall hypothesis that P/R-A and PIR-B play regulatory roles in inflammatory, coagulative, antigen-presenting, allergic, and humoral immune responses during host defense. This hypothesis will be tested in the following Specific Aims: 1) To identify the PIR ligands; 2) To determine the functional consequences of PIR-B deficiency in a gene-targeted mouse model; 3) To determine the molecular heterogeneity of PIR-A isoforms with respect to their glycosylation and association with adaptor proteins. The data generated in these mouse models show promise in establishing the foundation for the analysis of this type of regulatory mechanism in humans, and could suggest novel strategies for vaccine development, therapies for acute and chronic inflammatory responses, and treatments of allergic responses.